# CVE-2026-68398: ppp: defer channel free to an RCU grace period to fix pppol2tp RX UAF

> An unprivileged user can trigger a use-after-free in the Linux kernel PPP L2TP receive path, potentially leading to code execution or privilege escalation.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 10 Aug 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-68398, DEBIAN-CVE-2026-68398, GHSA-p572-44qq-qhm9, UBUNTU-CVE-2026-68398

## What is CVE-2026-68398?

CVE-2026-68398 is a high-severity use after free vulnerability in Linux, published on 10 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

ppp: defer channel free to an RCU grace period to fix pppol2tp RX UAF

pppol2tp_recv() runs in the L2TP UDP-encap softirq RX path:

 l2tp_udp_encap_recv() -> l2tp_recv_common() -> pppol2tp_recv()
   -> ppp_input(&po->chan)

It runs under rcu_read_lock() holding only an l2tp_session reference and
takes NO reference on the internal PPP channel (struct channel,
chan->ppp) that ppp_input() dereferences.

The pppox socket is SOCK_RCU_FREE, so 'po' and the embedded ppp_channel
are RCU-safe.  But the internal struct channel is a separate allocation
that ppp_release_channel() frees with a plain kfree():

 close(data socket) -> pppol2tp_release() -> pppox_unbind_sock()
   -> ppp_unregister_channel() -> ppp_release_channel() -> kfree(pch)

For a channel that is bound (PPPIOCGCHAN) but not attached to a ppp unit
(no PPPIOCCONNECT, pch->ppp == NULL) and not bridged, teardown skips
both ppp_disconnect_channel()'s synchronize_net() and
ppp_unbridge_channels()'s synchronize_rcu(), so the kfree() has no grace
period.  rcu_read_lock() in pppol2tp_recv() does not protect against a
plain kfree(), so an in-flight ppp_input() on one CPU can dereference
the channel just freed by close() on another CPU.

The bug is reachable by an unprivileged user.

Defer the channel free to an RCU callback via call_rcu() so the grace
period fences any in-flight ppp_input(). The disconnect and unbridge
teardown paths already fence with synchronize_net()/synchronize_rcu();
call_rcu() does the same here without stalling the close() path.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2026-68398?


## How do I fix CVE-2026-68398?

Apply the vendor's patch.

## Is CVE-2026-68398 being exploited?

There is no known exploitation of CVE-2026-68398 in the wild and no public exploit code.

Public exploit code:

- CVE-2026-68398 Ubuntu PPPoL2TP use-after-free local privilege escalation (github_poc): https://github.com/aramosf/CVE-2026-68398

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-68398 and fix it if it is affected.

What it is: CVE-2026-68398 is a high-severity use after free vulnerability in Linux, published on 10 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-68398 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-68398/ (Markdown: https://insights.sbomify.com/CVE-2026-68398/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-68398
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-68398
- OSV: https://osv.dev/vulnerability/CVE-2026-68398

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-68398/
Summaries are generated and may contain mistakes; the official description above is the record.